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UNIVERSITY  OF  ILLINOIS  BULLETIN 


Vol.  X.  5EPTEMBE.R  16,  1912.  No.3 

[Lntered  February  14,  1902,  at  Urbana,  Illinois,  as  second-class  matter  under 
Act  of  Congress  of  July  16,  1894.] 


BULLETIN  No.  17 
DEPARTMENT  OF  CERAMICS 

A.  V.  BLLININGER.  Director 


THE  EFFECT    OF  ACID5    AND  ALKALIE5  UPON 
CLAY  IN  THE  PLA5TIC  5TATE 

BY 

A.  V.  BLLININGER  AND  C.  E.  FULTON 


NOTE  ON  THE  DISSOCIATION  OF  CALCIUM 
HYDRATE 


BY 
R.  K.  HUR5H 


NOTE  ON  THE  RELATION  BETWEEN  PREHEAT- 
ING TEMPERATURE  AND  VOLUME 
SHRINKAGE 

BY 
R.  K.  HUR5H 

1911-1912 


PUBL15HLD  FORTNIGHTLY  BY  THE  UNIVERSITY 


[Reprinted  from  Transactions  American  Ceramic  Society.     \'<>I.  XIV. 
BY  Permission] 

THE  EFFECT  OF  ACIDS  AND  ALKALIES  UPON  CLAY  IN  THE 
PLASTIC  STATE. 
A.  V.  Bleixinger  and  C.  E.  Filton,  Urbana,  111. 
INTRODUCTION. 

The  effect  of  acids,  alkalies  and  salts  upon  clay  suspensions 
(slips)  has  been  discussed  frequently,  and  the  work  of  Simonis, 
Mellor,  Rieke,  Boettcher,  Ashley,  Foerster  and  Bollenbach  deals 
mth  the  viscosity  and  other  phenomena  of  systems  in  this  state. 
But  little  is  known  concerning  the  effect  of  such  reagents  upon 
clays  in  the  plastic  condition  which  differs  from  that  of  a  sus- 
pension, due  to  the  cohesive  inlluence  of  the  particles  upon  each 
other. 

It  has  been  realized  for  some  time  that  the  properties  of  clays 
in  the  wet  state  are  influenced  by  the  presence  of  alkalies  and 
acids.  Seger  explains  the  increase  in  the  plasticity  of  clay  upon 
storing  by  the  assumption  that  the  fermentation  of  organic  sub- 
stances results  in  acids  which  neutralize  the  alkalinity  due  to  the 
decomposed  feldspar,  and  in  addition  bring  about  the  "sour" 
condition  which  accompanies  the  improvement  in  working 
qualities.  Rohland'  discusses  this  subject  from  the  theoretical 
standpoint  and  makes  quite  definite  statements  with  reference 
to  the  principles  underlying  the  effect  of  various  reagents  upon 
clays  in  the  plastic  state.  He  arrives  at  the  conclusion  that  the 
plasticity  of  clays  is  increased  by  the  presence  of  H"*"  ions,  while, 
on  the  other  hand,  the  OH'  ions  are  active  in  the  opposite  direc- 
tion. According  to  Rohland,  the  plasticity  is  likewise  increased 
by  the  addition  of  colloids  like  tannin,  dextrine,  etc.,  as  has  been 
shown  by  the  work  of  Acheson,  fine  grinding  and  the  storage  of 
the  clay  in  cool  and  moist  places.  It  is  supposed  that  the  in- 
crease in  plasticity  is  coincident  with  the  coagulation  which  is 
primarily  due  to  the  presence  of  the  hydrogen  ions ;  it  is  retarded 
by  the  hydroxyl  ions.  The  salts  of  strong  bases  and  weak  acids 
which  dissociate  OH'  ions  hydrolytically  produce  an  effect 
similar  to  that  of  the  hydroxyl  ions.  Neutral  salts,  Rohland 
goes  on  to  say,  with  but  few  exceptions,  are  indifferent  in  their 


>  "Die  Tone,"  pp.  35-19. 


4         EFFECT  OF  ACIDS  AND  ALKALIES  UPON  CLAY. 

effect,  though  some  appear  to  show  a  contradictory  behavior, 
which  has  not  yet  been  explained.  "The  effect  of  the  hydroxyl 
ions  may  be  weakened,  compensated  or  strengthened  by  the  action 
of  the  salt  in  question.  Thus  borax  is  an  example  of  the  first 
class  and  sodium  carbonate  of  the  second." 

The  same  writer  further  says  that  with  some  clays  the  addi- 
tion of  NajCOa  brings  about  an  improvement  in  plasticity,  while 
ordinarily  the  same  reagent  behaves  in  the  opposite  sense,  due 
to  the  hydrolytic  dissociation  of  OH'  ions.  It  is  possible  that 
the  effect  of  hydroxyl  ions  might  be  neutralized  by  the  CO3" 
ions. 

DRYING  SHRINKAGE. 
A  decided  lack  of  data  exists  with  reference  to  the  deter- 
mination of  the  effect  of  reagents  upon  the  plasticity  of  clays. 
It  was  thought  advisable  for  this  reason  to  begin  work  along 
this  line  without  reference  to  any  theoretical  speculations.  The 
most  obvious  criterion  to  be  used  in  this  connection  is  the  drying 
shrinkage,  which,  from  what  we  know  of  the  properties  of  clays, 
is  a  function  of  plasticity.  It  is  evident  that  any  effect  caused 
by  the  addition  of  reagents  will  at  once  be  indicated  by  the 
shrinkage  of  the  clay. 

In  this  series  of  experiments  Georgia  kaolin  was  used.  This 
clay  was  found  to  show  an  acid  reaction  when  tested  with  phenol- 
phthalein.  This  would  indicate  that  the  addition  of  acid  should 
bring  about  no  decided  change  in  the  clay,  a  fact  which  was 
verified  by  experiment.  The  reagents  employed  were  HCl, 
H2SO4,  NaOH  and  Na2C03.  In  carrying  out  the  work  a 
thoroughly  mixed  sample  was  first  prepared  so  that  variations 
due  to  differences  in  composition  were  reduced  to  a  minimum. 
The  test  specimens  were  in  the  shape  of  bars  3V16  x  i  x  Vs  inches. 
Even  the  most  careful  linear  shrinkage  measurements  by  means 
of  the  vernier  caliper  were  found  to  be  unsuitable  for  the  work. 
A  volumenometer  permitting  of  readings  to  0.05  cc.  was  then 
employed.  The  measuring  liquid  used  was  petroleum  from  which 
the  lighter  oils  had  been  expelled  by  heating.  The  bars  were  at 
once  weighed  and  allowed  to  dry  at  the  laboratory  temperature 
for  three  days,  after  which  they  were  heated  at  110°  to  constant 


EFFECT   OF   ACIDS    AND   ALKALIES    UPON    CLAY.  5 

weight,  and  their  shrinkage  determined.     For  each  concentration 
of  reagent  three  bars  were  made  and  measured. 

Clay  and  Water. — A  study  was  first  made  of  the  drying 
shrinkage  of  the  clay  with  different  amounts  of  water,  ranging 
from  the  soft  state  in  which  the  clay  could  be  barely  molded 
to  the  condition  of  minimum  water  content  when  molding  was 
likewise  difficult  for  the  opposite  reason.  The  shrinkage  rela- 
tions to  the  various  contents  of  water  are  sho\vn  in  Fig.  i.  The 
third  point  on  the  curve,  shomng  a  shrinkage  of  10.45  per  cent, 
with  a  water  content  of  32.8  per  cent.,  represents  the  most  work- 
able state.  An)-  increase  in  water  above  this  point  is  at  once  ob- 
served by  the  rapid  softening  of  the  mass.  The  clay  hence  is  well 
suited  for  the  work  at  hand,  owing  to  the  ease  with  which  the 
condition  of  best  working  behavior  is  recognized  in  distinction 
from  many  other  plastic  clays  which  possess  a  long  working 
range. 

Effect  of  Acid. — Upon  adding  from  0.025  to  0.525  gram  of  hy- 
drochloric acid  to  100  grams  of  clay,  we  observe  from  Fig.  2  that  the 
shrinkage  is  not  materially  aflfected  by  this  reagent.  While  two 
maxima  of  somewhat  greater  contraction  are  noted,  the  principal 
result  seems  to  be  a  reduction  in  shrinkage,  contrary  to  what 
might  be  expected  from  Rohland's  statements.  The  fact  re- 
mains, however,  that  conditions  are  more  complex  than  they 
seem,  due  to  the  probable  solution  of  various  salts  in  the  clay 
as  well  as  the  formation  of  some  chlorides  by  the  acid. 

It  was  thought  that  further  insight  into  the  effect  of  the  acid 
might  be  obtained  by  calculating  the  total  and  the  shrinkage 
water  in  terms  of  the  true  clay  volume,  /.  c,  weight  divided  by 
the  density  of  the  powdered  substance,  according  to  the  rela- 
tion: 
100  (Vi  —  V2) 

■w  =  per  cent,  (by  volume)  shrinkage  water. 

1 
Where  x\  =  volume  of  wet  brickette, 
t'2  =  volume  of  dry  brickette, 
w  =  weight  of  brickette,  dried  at  110°  C, 
d  =  density  of  the  dry  and  powdered  clay. 


EFFECT  OF  ACIDS  AND  ALKALIES  UPON  CLAY. 


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BFFECT    OF   ACIDS   AND   ALKALIES   UPON    CLAY. 


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EFFECT    OF   ACIDS   AND   ALKALIES    UPOX    CLAY. 


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lO        EFFECT  OF  ACIDS  AND  ALKALIES  UPON  CLAY. 

Similarly,  the  volume  of  the  total  water  in  terms  of  the  true 
clay  volume  is  calculated. 

In  the  diagram  of  Fig.  3,  the  respective  volumes  of  total  and 
shrinkage  water  are  shown.  The  boundary  between  the  volumes 
of  water  and  that  of  clay  is,  of  course,  the  line  representing  zero 
water  and  100  volume  per  cent,  of  clay.  It  is  shown  in  Fig.  3 
that  the  content  of  pore  water  has  been  decreased,  that  of  the 
shrinkage  water  having  been  increased  both  at  the  expense  of  the 
pore  water  and  due  to  the  rise  in  the  total  water  content  at  the 
two  max.  points. 

The  addition  of  sulphuric  acid  likewise  tends  to  decrease  the 
shrinkage  as  is  shown  in  the  diagram  of  Fig.  4. 

Effect  of  Alkalies. — The  influence  of  NaOH  is  illustrated 
in  the  diagram  of  Fig.  5.  It  is  at  once  noted  that  with  0.2  per 
cent,  of  this  reagent  a  striking  max.  point  is  reached,  indicating 
a  marked  increase  in  shrinkage,  contrary  to  what  we  should  ex- 
pect according  to  Rohland's  views.  Only  after  adding  larger 
amounts  does  the  contraction  descend  towards  the  normal  value. 
Here  again,  according  to  Fig.  6,  the  increased  shrinkage  is  due  in 
part  to  the  specific  effect  of  the  reagent  in  increasing  the  distance 
between  the  particles  in  the  plastic  state  and,  in  part,  to  the 
denser  structure  of  the  clay  upon  drying.  Beyond  the  max. 
point  this  condition  changes,  since  the  pore  water  line  rises  above 
the  normal  level.  Since,  at  the  same  time,  the  total  water  line 
descends,  the  shrinkage  is  gradually  decreased.  The  structure 
of  the  dried  clay  is  thus  more  open  with  the  higher  contents  of 
NaOH  than  with  the  smaller  additions. 

The  growth  in  shrinkage  is  still  more  pronounced  in  the  case 
of  NajCOj,  Fig.  7,  a  phenomenon  contrary  again  to  Rohland's 
statements,  although,  of  course,  in  this  case  the  effect  of  the 
CO3  ion  might  have  proven  a  factor,  especially  if  absorption  has 
taken  place  to  any  appreciable  extent.  However,  even  under 
this  assumption,  it  is  somewhat  improbable  that  the  carbonic 
acid  could  have  brought  about  such  a  change  where  other  acids 
failed  to  accomplish  anything  like  the  same  result.  In  this 
diagram  the  maximum  occurs  with  0.7  per  cent,  of  the  reagent. 
With  larger  concentrations  the  shrinkage  is  again  reduced,  but 
appears  to  gain  once  more  with  amounts  beyond  1.2  per  cent. 


EFFECT    OF    ACIDS   AND    ALKAI.IICS    UPOX    CUAY.  I  I 

As  may  be  observed  from  the  diagram  of  Fig.  8,  the  pore  water 
volume  is  diminished  throughout  this  series  with  a  gradually 
increasing  total  water  content  up  to  the  maximum. 

DEFLOCCULATION  SERIES. 
It  was  thought  desirable  to  study  the  effect  of  the  acids  and 
alkalies  upon  the  clays  as  regards  dellocculation,  using  solutions 
of  the  same  concentration  present  in  the  plastic  clay,  as  sho\\Ti 
by  the  preceding  curves.  To  illustrate:  If  to  loo  grams  of  clay, 
requiring  34.9  per  cent,  of  water,  0.025  gram  Na^COj  was  added, 
this  would  represent  a  solution  carrying  0.025  "^  34-9  =  0.000716 
gram   NaoCOj  per  cubic  centimeter  of  water.      Such    solutions 


Fig.  9. 


No. 

Wt.  clay. 
Grams 

wt.  NajCOa, 
Grains 

Water, 
cc. 

Volume  of 

sediment, 

cc. 

Condition  of 

turbidity  of 

supernatant  liquid 

0 

5 

98 

18.0 

Clear 

I 

5 

0.0713 

98 

19-5 

" 

2 

5 

0.1423 

98 

21.8 

" 

3 

5 

0.2296 

98 

24  3 

» 

4 

5 

0.4529 

98 

28.0 

u 

5 

5 

0 . 6803 

98 

-^7-4 

n 

6 

5 

0.8913 

98 

28.0 

« 

7 

5 

1.0659 

98 

28.0 

» 

8 

5 

1-3549 

98 

28.0 

« 

9 

5 

I   55" 

98 

28.0 

u 

12 


EFFECT    OF    ACIDS   AND   ALKALIES    UPON    CLAY. 


were  made  up  of  concentrations  corresponding  to  the  various 
points  in  the  preceding  curves.  In  each  case  to  5  grams  of  clay 
98  cc.  of  the  solution  were  added  in  a  graduated  tube.  The 
tubes  were  placed  in  a  shaking  machine  for  90  minutes  and 
allowed  to  stand.  It  was  found  that  the  clay  itself,  without  any 
reagent,  settled  well,  showing  a  clear,  supernatant  liquid  and  a 
sediment  occupying  18  cc. 

It  was  shown  that  the  addition  of  acid  produced  no  change, 
excepting  in  the  volume  of  the  sediment,  which  was  finally  in- 
creased from  18  to  28  cc,  as  is  observed  from  Fig.  9. 

The  sodium  carbonate  solutions,  on  the  other  hand,  started 
with  conditions  of  complete  deflocculation  (Fig.  10).  The  sediment 
volumes  are  shown  in  the  table  accompanying  each  figure. 


m 

<^'  ^^1  flii 

Fig.  10. 


No. 


Wt. 
clay, 
grams 

wt.  NaaCOs, 

Water, 

Volume  of 

sediment, 

cc. 

Condition  of  turbidity 

grams 

cc. 

supernatant  liquid 

5 

98 

18 

Clear 

5 

0.0702 

98 

4-5 

Very  turbid 

5 

0. 1426 

98 

19 

Very  turbid 

5 

0 . 2090 

98 

24 

Slightly  less  turbid 

5 

0.2822 

98 

24 

Less  turbid 

5 

0-5713 

98 

20 

Slightly  turbid 

5 

0.8388 

98 

20 

Slightly  turbid 

5 

1.3642 

98 

21 

Almost  clear 

5 

1-8571 

98 

19 

Clear 

5 

2-5059 

98 

18 

Clear 

5 

3 . 4006 

98 

18 

Clear 

5 

4.0709 

98 

18 

Clear 

o. 

I . 

2  . 

3- 
4 
5- 
6. 

7  ■ 
8. 

9 
10. 


EFFECT    OF   ACIDS   AND    ALKALIES    UPON    CLAY.  I3 

The  maximum  point  of  the  shrinkage  curve  corresponds  to 
tube  No.  8,  where  the  supernatant  Hquid  is  clear  for  the  first 

time. 

CONCLUSIONS. 

The  writers  do  not  attempt  at  this  time  to  explain  the 
phenomena  on  theoretical  grounds.  It  is  evident  that  the  con- 
ditions are  quite  complex  and  in  order  to  explain  them  still  further 
modes  of  attack  must  be  sought  for.  The  rules  laid  down  by 
Rohland  do  not  seem  to  apply,  since  in  the  main  the  acids  cleaily 
caused  shrinkage  to  decrease  while  the  alkalies  produced  the 
reverse  effect,  which  is  contrary  to  his  statements.  In  order  to 
be  fair,  however,  attention  must  be  called  to  the  fact  that  shrinkage 
in  this  work  has  been  considered  a  meastu-e  of  plasticity,  while 
Rohland  speaks  of  plasticity  itself  without  attempting  to  correlate 
this  property  with  any  numerical  value.  As  is  well  known,  there 
is  at  the  present  time  no  clear  conception  as  to  the  relation  be- 
tween plasticity  and  shrinkage  excepting  the  general  fact  that 
the  plastic  clays  as  a  class  show  a  greater  dr>dng  shrinkage  than 
the  leaner  ones. 

DISCUSSION. 
Mr.  R.  J.  Montgomery:     I  should  like  to  ask  Prof.  Bleininger 
how  long  those  slips  in  the  cylinders  had  stood  when  the  photo- 
graphs were  taken. 

Prof.  Bleininger:  Twenty-four  hours.  I  might  add  also 
that  in  making  the  volume  determinations  they  were  stored 
twelve  hours  in  a  moist  chamber  in  order  to  bring  about  some  sort 
of  an  equilibrium  between  the  clay  and  the  reagent. 

Mr.  Kerr:  I  should  like  to  raise  the  question  as  to  what 
determinations,  if  any,  were  made  of  the  electrolytes  present 
in  the  clay  before  the  acids  and  alkalies  were  added.  Was  any 
general  data  obtained  upon  this  point? 

Prof.  Bleininger:  No  direct  determination  was,  of  course, 
made.  However,  you  have  seen  the  series  of  tubes  which  ought 
to  indicate  pretty  clearly  to  one  familiar  with  this  work  whether 
the  initial  conditions  are  acid  or  alkaline.  We  are  principally 
endeavoring  to  get  at  the  experimental  facts  without  much  re- 
gard to  theoretical  assumptions.  The  evidence  so  far  obtained 
along  these  lines  is  not  sufficient  to  base  upon  it  any  definite 


O.  0^  {LL  Ub. 


14  EFFECT    OF   ACIDS    AND    ALKALIES   UPON    CLAY. 

line  of  procedure.  The  work  of  VeimaTn  especially  has  dis- 
turbed pievious  conclusions  by  his  very  startling  claims  with 
reference  to  colloids.  We  thought  it  wise  to  work  along  the  lines 
which  I  have  indicated. 

Mr.  Kerr:  The  only  point  which  I  wished  to  bring  up  was 
that  if  one  clay  contained  positive  ions  in  excess  and  another  clay 
negative,  the  addition  of  either  acid  or  alkali  to  one  clay  would 
not  correspond  to  a  similar  addition  to  the  other  clay.  Some 
clays  give  a  strongly  acid  reaction,  others  a  weakly  acid,  while 
still  others  are  somewhat  alkaline.  Data  upon  neutralization 
might  be  included. 

Prof.  Bleimnger :  This  is  brought  out  in  the  deflocculation 
experiments.  At  the  same  time  corrections  work  very  well  in 
theors^  but  when  you  come  to  make  them  you  will  find  that 
neutralization  does  not  necessarily  follow.  I,  of  course,  want  to 
check  Mr.  Ashley's  work  in  this  investigation  in  a  general  way. 
I  realize  we  have  learned  a  good  deal  from  his  work  and  I  want 
to  say  that  he  is  to  be  given  great  credit  for  having  started  work 
of  this  kind. 

Mr.  Purdy:  I  would  like  to  ask  if  any  experiment  has  been 
made  to  determine  whether,  as  a  rule,  trivalent  electrolytes  coagu- 
late clays  more  readily  than  do  the  uni-  and  divaleat  salts. 

Prof.  Bleininger:  I  would  say  that  it  has  been  done  with 
various  materials. 

Mr.  Purdy:  Has  it  been  done  with  clays?  I  would  like  to 
see  some  experiments  tried  on  that  and  reported,  because  I  have 
been  unable  to  show  that  the  trivalent  salts  have  any  more  effect 
than  the  other.  That  is  one  of  the  respects  in  which  the  clay  is 
different. 

Prof.  Bleininger:     Mr.  Ashley,  of  course,  has  done  such  work. 

Mr.  Purdy:  That  is  what  he  did  not  do,  he  accused  himself 
■on  that  point. 

Prof.  Bleininger:  I  think  he  did  work  with  phosphates.  Of 
course,  as  I  said  before,  this  work  is  being  continued  and  we 
expect  to  take  representative  reagents. 

Mr.  Kerr:  What  meastuements  other  than  volume  shrinkage 
were  made? 

Prof.   Bleininger:     We  hope  to  take  up  various  things  in 


EFFECT    OF   ACIDS   AND   ALKALIES    UPON    CLAY.  1 5 

time.  One  of  them  is  a  vapor  tension  investigation,  for  which 
a  special  apparatus  is  now  being  designed. 

Prof.  Grout:  I  would  like  to  ask  if  the  curves  which  are 
drawn  there,  such  as  the  first  curve  which  you  show  on  the  screen, 
were  the  average  of  a  series  of  results  on  one  clay  or  just  one 
series  of  tests. 

Prof.  Bleininger :  Taken  as  the  average  of  three  determina- 
tions in  each  case. 

Prof.  Grout:  I  wondered  if  that  approximation  of  a  maxi- 
mum was  so  characteristic  that  you  could  report  it  for  publica- 
tion on  one  series  of  tests;  whether  your  area  of  determination 
was  not  such  that  you  might  not  safely  report  it. 

Prof.  Bleininger:  Well,  we  were  able  to  get  very  good 
checks,  also  we  notice  that  the  two  acids  are  behaving  very 
similarly.  We  recognize,  however,  that  there  are  a  good  many 
factors  involved  which  it  is  almost  impossible  to  correlate  in  a 
technical  investigation  of  this  kind.  Of  course,  if  we  were  to 
carr}^  on  this  investigation  from  a  strictly  physical  chemical 
standpoint,  we  would  proceed  along  somewhat  different  lines. 

Mr.  Potts:  I  would  like  to  ask  Prof.  Bleininger  just  what 
practical  application  he  expects  to  make  of  that  treatment. 
Does  he  propose  to  make  kaolins  plastic? 

Prof.  Bleininger:  I  haven't  any  idea  as  to  what  this  in- 
formation could  be  used  for  and  am  indifferent  in  regard  to  that 
point. 


[Reprinted  from  Transactions  American  Ceramic  Society.     Vol.  XIV, 
BY  Permission.] 

NOTE  ON  THE  DISSOCIATION  OF  CALCIUM  HYDRATE. 

By  R.  K.   HuRSH. 

INTRODUCTION. 

The  present  study,  which  was  intended  to  be  of  a  techno- 
logical rather  than  of  physical-chemical  nature,  was  undertaken 
with  the  purpose  of  learning  more  regarding  the  properties  and 
behavior  of  the  compound  CaCOH),.  The  work  has  a  practical 
bearing  in  demonstrating  the  value  of  methods  of  thermal  study 
upon  problems  dealing  with  the  dehydration  of  limes,  cements 
and  plasters. 

A  number  of  values  have  been  given  for  the  dissociation 
temperature  of  calcium  hydrate.  Herzfeld^  says  that  dissocia- 
tion evidently  begins  at  470°  to  500°  C.  He  gives  the  thermal 
effect  of  slaking  CaO  as  i  .51  cals.  per  gram  of  CaCOH),  and  the 
maximum  temperature  of  formation  as  468°.  H.  Rose"-  found 
that  pure  calcium  hydrate  lost  nothing  at  100°  C,  absorbed  CO, 
at  200°  and  300°,  and  began  to  lose  H.p  at  about  400°  C. 

Le  Chatelier^  gives  a  vapor  tension  of  100  mm.  at  350°  C, 
and  760  mm.  at  450°  C. 

Tichborne^  found  the  precipitate  from  a  heated  solution  of 
lime  water  to  show  a  loss  on  blasting  that  corresponded  to  the 
formula  3Ca0.2H20.  Others  using  similar  methods  failed  to 
find  such  a  hydrate. 

Dr.  Johnston,^  whose  work  is  taken  up  further  on,  found 
the  dissociation  pressure  of  Ca(OH).,  to  reach  760  mm.  at  547°  C. 

METHODS  AVAILABLE. 
There  are  several  methods  of  studying  the  dissociation  of 
hydrates,  such  as  the  making  use  of  heating  curves,  the  deter- 
mination of  the  aqueous  pressure  in  direct  or  differential  tensim- 
eters,  and  the  method  depending  upon  the  determination  of 
the  loss  of  weight  at  different  temperatures 


'  llandbuch  dcr  anorg.  Chem.,  C.  Damnier. 

-  Pogg.  Ann.  du  Physik  u.  Chem..  LXXXVI.  105. 

<•  Handbuch  dcr  anorg.  Chem.,  22,  Gmelin  Kraut. 

<  Chemical  Xews.  XXIV,  199. 

5  Ztschr.    phys.  Chem..  I.XII,  330. 


l8  NOTE    ON    DISSOCIATION    OF    CALCIUM   HYDRATE. 

HEATING  CURVE  METHOD. 

A  portion  of  the  substance  is  placed  in  a  furnace  with  a 
thermocouple  touching  it  and  another  near  it.  The  furnace  is 
heated,  and  the  temperatures  of  the  furnace  and  substance  are 
noted.  At  the  point  where  dissociation  takes  place,  a  lag  may  be 
noted  in  the  heating  curve  due  to  the  endothermic  reaction,  /.  e., 
the  absorption  of  heat  due  to  the  expulsion  of  water.  It  is  fre- 
quently difficult  and  sometimes  impossible  to  determine  the  point 
by  this  means,  owing  to  the  small  amount  of  heat  required  for 
the  reaction  of  the  slow  rate  of  dissociation.  Distinction  may 
be  made  between  mechanically  held  or  dissolved  water  and  chem- 
ically combined  water.  In  the  case  of  chemical  water,  the  lag 
will  occur  abruptly  at  the  temperature  of  dissociation.  Mechan- 
ical or  dissolved  water  will  pass  off  gradually  over  a  range  of  tem- 
perature, and  the  lag  due  to  these  is  gradual,  showing  no  abrupt 
break  at  a  definite  temperature. 

In  the  use  of  heating  curves,  close  regulation  of  the  tempera- 
ture is  very  necessary  to  get  reliable  results.  There  should  be 
no  fluctuations  in  the  heating  of  the  furnace.  Three  general 
methods  may  be  followed  in  the  heating: 

Indiscriminate,  in  which  no  attention  is  given  to  the  rate  of 
the  furnace  curve,  and  only  the  lags  in  the  heating  curve  of  the 
substance  are  given  attention. 

Constant  rate,  in  which  the  temperature  of  the  furnace  is- 
raised  at  a  uniform  rate. 

Constant  difference,  in  which  a  uniform  difference  between 
furnace  temperature  and  that  of  the  material  is  maintained. 
This  method  is  the  best,  although  the  most  difficult  one  of  the 
three.  The  constant  rate  method  gives  good  points,  but  the  lag 
will,  in  most  cases,  be  sloped  instead  of  horizontal. 

AQUEOUS  PRESSURES  METHOD. 

Van  Bemmelen,  in  studying  the  dehydration  of  the  silicic 
acid  gel,  placed  his  samples  in  desiccators  containing  various 
concentrations  of  HjSO^.  Constant  temperature  was  maintained, 
and  the  samples  were  kept  in  the  desiccators  for  sufficient  time 
to  reach  equilibrium  imder  the  various  vapor  tensions.  By 
plotting  the  loss  of  weight  curve  for  the  several  concentrations 
of  H2SO4  or  the  corresponding  vapor  pressures,  he  was  able  to 


NOTE    ON    DISSOCIATION    OF   CAIXIl'M    HYDRATE. 


19 


determine  the  inversion  points  and  the  degrees  of  hydration  in 
each  case.  The  same  method  has  been  appUed  by  Prof.  A.  W 
Bleininger"  in  studying  the  moisture  in  clays. 

Dr.  John  Johnston^  studied  the  dissociation  pressures  of 
several  metal  hydroxides  and  carbonates,  using  two  experimental 
methods.  The  first  was  applied  for  hydroxides  alone  and  is  sim- 
ilar to  one  used  by  Brill.  A  small  crucible  containing  a  weighed 
portion  (about  i .  5  mg.)  of  the  substance  was  suspended  in  a  small 
electric  furnace  through  which  a  current  of  air  free  from  CO,  and 
of  definite  vapor  pressure  was  passed.  The  air  was  freed  from 
COo  by  passing  through  XaOH,  then  saturated  with  moisture  by 
bubbling  through  a  Liebig  potash  bulb,  containing  water,   and 


rf?^A^3.  ^/^.  c^/?.  soc.  /^22/  x/i^      y^/C?  ■  /. 


/H'O'^3/^ 


760 


'<3iS(P 


■4^C  <«c5i?  ^(P^ 


^s^o 


"  Bulletin  No.  7,  Bureau  of  Standards. 
'  Ztschr.  phys.  Chem.,  LXII.  p.  330. 


20  NOTE    ON    DISSOCIATION    OF    CALCIUM   HYDRATE. 

was  heated  before  passing  to  the  furnace  to  prevent  any  conden- 
sation. The  temperature  of  the  water  in  the  Liebig  bulb  was 
regulated  by  immersing  it  in  a  water  bath.  The  furnace  was 
held  at  constant  temperature,  and  the  vapor  tension  maintained 
at  a  definite  value  for  lo  minutes  by  regulation  of  the  water 
bath  temperature.  The  crucible  was  then  removed  from  the 
furnace  and  weighed  on  a  very  fine  balance.  Conditions  of  tem- 
perature and  vapor  pressure  were  so  regulated  that  the  substance 
maintained  constant  weight  or  gained  slightly  during  the  period, 
and  these  values  were  taken  as  the  corresponding  temperature 
and  dissociation  pressure  of  the  material.  The  results  of  this 
method  for  Ca(0H)2  are  shown  in  Fig.  i. 

This  method  was  found  to  be  too  slow  and  to  frequently 
give  inconsistent  results.  It  was  impossible  to  prevent  the  ab- 
sorption of  some  CO2  while  removing  the  crucible  from  the  fur- 
nace for  weighing. 

STATIC  METHOD. 

Dr.  Johnston  then  resorted  to  the  "static  method,"  in  which 
the  dissociation  pressirres  are  measured  directly.  A  diagram  of 
the  apparatus  is  showTi  in  Fig.  2.  A  platinum  tube,  P,  about  5 
cm.  long  and  4  mm.  inside  diameter,  contained  the  substance. 
This  tube  was  placed  in  a  small  electric  furnace  with  a  thermo- 
couple for  determining  the  temperatures.  A  piece  of  glass  tube, 
C,  was  fused  to  P  and  to  one  arm  of  a  U  tube  which  was  connected 
to  the  barometer.  On  each  arm  of  the  U  tube  was  a  bulb,  L, 
bent  to  the  side  and  holding  enough  mercury  to  fill  the  U-tube 
to  a  depth  of  about  3  cm.  To  prevent  condensation  of  the  vapor 
from  P,  the  U-tube  and  C  were  enclosed  by  a  glass  steam  jacket. 

With  the  mercury  in  bulb  L,  the  apparatus  was  exhausted 
through  A  by  means  of  a  mercur}'  pump.  Cock  A  was  then  closed, 
and  the  mercury  run  from  L  into  the  U-tube  by  tilting  the  ap- 
paratus. Heating  was  begun,  and  at  the  first  indication  of  pres- 
sure in  P,  the  mercur>^  in  the  two  arms  of  the  U-tube  was  brought 
to  the  same  level  by  admitting  some  air  at  B  and  adjusting  by 
means  of  the  leveling  tube  R. 

In  his  work  with  calcium  hj'droxide.  Dr.  Johnston  slaked 
pure  CaO  and  absorbed  the  excess  water  in  a  desiccator.  An- 
other portion  was  made  by  allowing  the  CaO  to  absorb  moisture 


NOTE    ON    DISSOCIATION    OF   CALCIUM    HYDRATE. 

r/?^/v3.  y^M.  dT^/f?  30C.  /^z.  x/P'  /ycz/F-s/-/ 


21 


slowly  until  the  composition  was  about  CaO  o.SH^O.  In  using 
this  substance,  it  was  found  necessary  to  heat  it  slightly  during 
exhaustion  of  the  apparatus  since  pressures  of  several  cm.  ap- 
peared between  200°  and  300°  which  again  disappeared  in  part 
on  further  heating.  These  abnormal  pressures  were  supposedly 
due  to  loosely  combined  or  absorbed  moisture,  and  upon  their 
appearance  the  test  was  stopped  and  the  apparatus  again  ex- 
hausted. Only  such  pressures  were  taken  as  appeared  at  definite 
temperatures  on  heating  and  again  disappeared  on  cooling.  The 
"abnormal  pressures"  disappeared  only  partly  on  cooling.  Un- 
der these  conditions,  it  was  found  advisable  to  use  a  mixture  of 
CaO  and  the  hydroxide,  although  this  did  not  entirely  eliminate 


22 


NOTE    ON    DISSOCIATION    OF    CALCIUM    HYDRATE. 


the  trouble,  which  was  noted  with  all  of  the  hydroxides  studied. 
The  results  of  the  work  on  Ca(0H)2  by  this  method  are  shown  in 
Fig.  3.     By  the  curve,  it  is  seen  that  the  dissociation  pressure 


eoa 


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reaches  760  mm.  at  a  temperature  of  547°  C.  Hence  this  is 
taken  as  the  dissociation  temperature  of  the  substance  under 
atmospheric  pressure. 

In  studying  zeolites  FriedeP  heated  them  at  successively 
higher  temperatures  in  a  current  of  air  of  approximately  constant 
vapor  pressure.  This  method  was  adopted  by  Allen  and  Clement'' 
in  their  study  of  tremolite,  using  dry  instead  of  moist  air.  A 
crucible  containing  the  material  was  placed  in  an  electric  fur- 
nace, through  which  a  current  of  air,  dried  by  concentrated 
H2SO4,  was  passed.  After  heating  for  some  time  at  a  definite 
temperatm"e,  the  crucible  was  quickly  removed  to  a  desiccator, 
cooled  and  weighed.  Heating  was  continued  at  each  tempera- 
ture until  practically  constant  weight  was  obtained.  In  one 
case,  the  experiment  was  repeated  with  moist  air  to  determine 
the  effect  upon  the  results  obtained  by  using  dry  air. 

EXPERIMENTAL  WORK. 

This  method  was  adopted  for  the  present  work.  The  hy- 
drate was  prepared  by  calcining  pure  CaCOg  at  1050°  C.  and 
slaking  the  oxide  with  a  slight  excess  of  water.  A  portion  of 
the  hydrate  was  placed  in  a  platinum  crucible  and  heated  in  an 


8  Ztschr.  phys.  Chem.,  XXVI,  p.  323. 
8  Am.  Jour.  Sci..  Vol.  XXVI.  No.  152. 


NOTE    ON    DISSOCIATION    OF    CALCIUM    HYDRATE. 


23 


electric  furnace  in  a  bath  of  dry  air,  free  from  CO,,  at  successive 
temperatures  from  200°  to  750°  C.  at  50°  intervals.  At  30-min- 
ute  intervals,  the  crucible  was  removed  from  the  furnace  and  cooled 
in  a  desiccator  over  concentrated  H^SOj.  The  heating  was  con- 
tinued at  each  temperature  until  constant  weight  was  reached. 
It  was  impossible  to  prevent  the  absorption  of  some  COj  during 
the  transfer  of  the  hot  crucible  from  the  furnace  to  the  desiccator. 
The  first  loss  of  weight  was  noted  at  400°  C.  Continued  heating 
at  this  temperature  gave  a  total  loss  of  weight  of  77  per  cent,  of 
the  water  present  above  200°  C.  At  650°  another  loss  of  weight 
took  place  amounting  to  22  per  cent.  A  second  trial  was  made 
with  10°  intervals  from  350°  to  400°  C,  and  the  first  loss  was 
found  to  take  place  at  380°  C. 

To  prevent  the  absorption  of  COj  by  the  sample,  a  method 
of  weighing  within  the  furnace  was  adopted.  A  platinum  cru- 
cible was  suspended  in  the  furnace  by  a  fine  platinum  wire  from 
one  pan  of  a  balance  that  was  carefully  protected  from  unequal 
heating  from  the  furnace.  A  thermocouple  was  placed  with  the 
junction  just  under  the  middle  of  the  crucible.  A  current  of 
air,  free  from  COj  and  dried  by  CaClj  and  P2O5,  was  circulated 
through  the  furnace.  A  weighed  portion  of  the  hydrate  was 
placed  in  the  crucible  and  dried  at  25°  C.  The  temperature  was 
then  raised  gradually  until  a  loss  of  weight  began  at  375°  C. 
It  was  somewhat  surprising  that  there  was  no  loss  of  weight  be- 


r/j>j/KS.>»/K  cf-zp  sec  t^OZ.   -^/i^ 


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24 


NOTE    ON    DISSOCIATION    OF    CALCIUM    HYDRATE. 


tween  25°  and  375°  as  some  mechanically  held  water  might  be 
expected.  After  constant  weight  was  reached  at  375°,  the 
heating  was  continued  beyond  the  point  noted  by  Johnston. 
The  second  loss  of  weight  took  place  at  580°  C.  The  loss  of  weight 
curves  for  several  trials  are  shown  in  Fig.  4. 

To  determine  whether  the  presence  of  moisture  in  the  fur- 
nace would  have  any  effect  upon  the  results,  the  air  current  was 
saturated  at  0°  to  1°  C.  before  passing  through  the  furnace, 
giving  a  vapor  pressure  of  about  5  mm.  The  loss  of  weight  was 
found  to  occur  at  the  same  points  as  before  but  to  proceed  at  a 
slower  rate.  The  quantitative  results  diflfer  somewhat,  due 
possibly  to  the  longer  time  required  in  the  latter  trial.  The  re- 
sult of  the  trial  is  shown  in  Fig.  5. 


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The  results  of  these  experiments  indicate  the  existence  of 
two  hydrates  of  CaO,  the  CaO.H20  dissociating  at  375°  C,  leav- 
ing a  lower  hydrate  which  dissociates  at  580°  C,  leaving  CaO. 
That  the  loss  of  weight  at  each  point  is  due  to  the  dissociation 
of  a  chemical  compound  is  shown  by  the  shape  of  the  curves. 
The  break  is  abrupt  with  no  gradual  slope  preceding  it.  If  me- 
chanical or  dissolved  water  were  being  driven  off,  there  would 
be  a  gradual  loss  of  weight  with  increasing  temperature. 

SUMMARY. 
Various  temperatures  are  given  for  the  dissociation  of  Ca(0H)2 
ranging  from  450°  by  Le  Chatelier  to  547°  C.  by  Johnston. 


NOTE    ON    DISSOCIATION    OF    CALCIUM    HYDRATE.  25 

Using  the  "loss  of  weight"  method,  two  dissociation  points 
are  found.  The  hydrate,  Ca(0H)2,  dissociates  at  375°,  forming 
a  lower  hydrate  that  loses  its  HJJ  at  580°  C. 

No  mechanical  water  was  driven  off  above  25°  C. 

In  conclusion,  the  writer  wishes  to  express  his  indebtedness 
to  Professor  A.  Y.  Bleininger  for  many  valued  suggestions  in  this 
work. 


[Reprinted  from  Transactions  American  Ceramic  ?ociety.     Vol.  XIV, 
BY  Permission.] 

NOTE  ON  THE  RELATION   BETWEEN   PREHEATING   TEM- 
PERATURE AND  VOLUME  SHRINKAGE. 

By  R.  K.  HuRSH. 

INTRODUCTION. 

An  extended  study  of  the  effect  of  preliminary  heat  treat- 
ment upon  clays  within  a  practical  temperature  range  has  been 
made  by  Professor  Bleininger.*  Especial  attention  was  given  to 
the  effect  upon  the  volume  shrinkage.  A  decided  change  in  the 
properties  of  most  of  the  clays  was  noted  at  temperatures  of  200° 
to  300°  C.  They  became  more  or  less  granular  and  decreased 
markedly  in  plasticity.  There  was  a  material  decrease  in  the 
volume  shrinkage  and  an  increase  in  the  amount  of  pore  water. 
In  a  few  cases,  this  change  occurred  at  somewhat  higher  tempera- 
tures. One  fine-grained,  highly  plastic  clay,  similar  in  behavior 
to  bentonite,  showed  a  considerable  change  in  physical  proper- 
ties at  250°;  but  treatment  at  temperatures  up  to  400°  failed  to 
reduce  the  shrinkage  to  w^orking  limits. 

Professor  Or  ton,-  in  studying  some  tertiary  clays  which  gave 
trouble  in  drying,  found  ordinary  preheating  temperatures  to 
be  ineffective.  When  the  temperature  was  raised  to  450°-5io° 
C.  the  plasticity  and  shrinkage  were  reduced  sufficiently  to  make 
the  clay  workable.  Under  the  conditions  of  the  tests  the  period 
of  safe  treatment  at  these  temperatures  was  closely  limited  since 
the  clays  lost  their  plasticity  entirely  when  kept  a  little  too 
long  in  the  dryer.  As  some  time  is  required  for  heat  to  penetrate 
the  clay  it  is  possible  that  the  temperature  may  have  reached 
a  higher  point  in  the  longer  treatments  than  was  indicated  by 
the  thermo-couple.  The  test,  however,  represents  the  prac- 
tical conditions  in  a  rotary  dryer. 

EXPERIMENTAL  WORK. 
The  present  work  was  undertaken  with  the  purpose  of  se- 
curing some  further  data  upon  the  effect  of  the  higher  tempera- 
tures of  preheating  upon  the  physical  properties  of  clays  as  in- 
dicated by  changes  in  volume  shrinkage.     Four  clays  were  used: 


1  Bull.  No.  7.  Bureau  of  Standards. 

2  Trans.  A.  C.  S..  Vol.  XIII,  p.  765. 


28        PREHEATING   TEMPERATURE    AXD    VOLUME    SHRINKAGE. 


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PREHEATING    TEMPERATURE    AN'D    VOLUME    SHRINKAGE.  29 

No.  I.  A  plastic,  somewhat  sandy  surface  clay  from  Urbana, 
Illinois. 

No.  2.     A  plastic,  red-burning  shale  from  Danville,  111. 

No.  3.  A  plastic,  No.  2  fire  clay,  having  a  high  drying 
shrinkage,  from  St.  Louis,  Mo. 

No.  4.  A  fine  grained,  weathered  shale  from  Saskatchewan 
similar  in  character  to  the  clays  studied  by  Professor  Orton. 
It  became  very  sticky  in  the  plastic  state  and  cracked  to  pieces 
under  any  conditions  of  drying.  It  is  of  interest  to  note  that  the 
addition  of  i  per  cent,  of  NaCl  greatly  improved  the  working 
properties  and  reduced  the  drying  shrinkage  nearly  one-half. 
It  would  be  possible  by  this  treatment  to  make  commercial  use 
of  the  material. 

The  clays  were  heated  at  temperatures  50°  apart  from  250° 
to  650°  C.  for  three  hours,  from  i  to  2  hours  being  required  to 
reach  the  temperature.  They  were  then  ground  to  pass  20  mesh 
and  made  up  into  small  briquets.  These  were  weighed  and  the 
volumes  measured,  dried  in  air  and  at  110°  in  an  oven,  weighed, 
immersed  in  coal  oil  for  several  hours  and  the  dry  volumes  meas- 
ured. Care  was  taken  to  get  about  the  same  consistency  in  the 
samples  when  making  up  the  briquets.  The  shrinkage  curves 
are  sho\vn  in  Fig.   i  and  the  moisture  content  in  Fig.  2. 

DISCUSSION  OF  RESULTS. 

The  surface  clay.  No.  i,  changed  in  color  from  yellow  to 
brown  at  250°  and  to  a  light  salmon-red  at  400°.  The  plasticity 
was  considerably  decreased  at  250°,  was  very  low  at  400°,  the 
briquets  being  very  friable,  and  was  entirely  gone  at  450°.  The 
color  changes  seem  to  correspond  closely  to  the  changes  in  vol- 
ume shrinkage.  Above  300°  the  shrinkage  decreased  rapidly  to 
400°,  beyond  which  the  heat  treatment  had  little  efi"ect. 

The  shale.  No.  2,  changed  from  gray  to  brown  at  350°  and  to 
red  at  400°.  The  plasticity  was  considerably  decreased  at  200°, 
but  decreased  gradually  from  200°  to  600°.  At  650°  no  plasticity 
remained,  and  the  briquets  were  too  fragile  to  handle.  The  shrink- 
age decreases  very  little  from  200°  to  550°  but  drops  considera- 
bly at  600°. 

The  fire  clay,  No.  3,  decreased  in  plasticity  gradually  up  to 


30  PREHEATING   TEMPERATURE   AND   VOLUME    SHRINKAGE. 

400°,  but  at  450°  it  became  buff  in  color  and  was  practically 
non-plastic.  The  effect  of  the  heat  treatment  is  much  more 
marked  than  with  the  surface  clay  and  the  shale.  The  shrink- 
age curve  drops  very  abruptly  at  350°.  The  behavior  of  this 
clay  is  similar  to  that  of  an  English  ball  clay  studied  by  Professor 
Bleininger.^ 

The  weathered  shale,  No.  4,  changed  in  color  from  gray  to  deep 
maroon  at  330°,  at  which  point  the  cracking  of  the  briquets  was 
noticeably  decreased  but  was  still  very  bad.  Cracking  decreased 
gradually  beyond  this  temperature,  but  the  briquets  at  550° 
were  the  only  ones  that  remained  sound  with  open-air  drying. 
The  sticky  quality  of  the  clay  was  retained  up  to  500°.  At  550° 
it  was  quite  granular  but  developed  considerable  plasticity  with 
wedging.  The  effect  of  the  heating  treatment  upon  the  shrink- 
age is  more  pronounced  than  with  the  other  clays,  but  an  ab- 
normally high  shrinkage  remains  500°.  Beyond  this  point  the 
drop  in  the  curve  is  so  abrupt  that  very  careful  temperature  con- 
trol would  be  necessary  in  obtaining  a  sufficient  reduction  in 
shrinkage  to  prevent  cracking  without  destroying  the  working 
properties.  From  the  high  temperature  required  and  the  narrow 
range  of  safe  heat  treatment,  it  is  obvious  that  preheating  would 
not  be  a  safe  method  for  practical  use  with  such  a  clay. 

The  effect  of  the  heat  treatment  upon  these  clays  is  quite 
different.  The  shale  is  most  gradually  affected,  losing  its  plas- 
ticity entirely  only  at  temperatures  above  red  heat.  It  is  proba- 
ble that  this  is  characteristic  of  the  more  homogeneous  materials. 

The  fire  clay  shows  an  abrupt  drop  in  its  shrinkage  curve, 
behaving  similarly  to  other  fire  clays  and  a  certain  ball  clay. 

The  fourth  clay  has  such  abnormally  high  shrinkage  that 
only  treatments  above  500°  C.  would  suffice  to  eliminate  crack- 
ing in  drying.  It  is  evident  that  clays  of  this  type  are  not  adapted 
to  preheating  treatment. 


